1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, wireless (contactless) power transmission has been receiving attention as a power supply technique for electronic devices such as cellular phone terminals, laptop computers, etc., or for electric vehicles. Wireless power transmission can be classified into three principal methods using an electromagnetic induction, an electromagnetic wave reception, and an electric field/magnetic field resonance.
The electromagnetic induction method is employed to supply electric power at a short range (several cm or less), which enables electric power of several hundred watts to be transmitted in a band that is equal to or lower than several hundred kHz. The power use efficiency thereof is on the order of 60% to 98%.
In a case in which electric power is to be supplied over a relatively long range of several meters or more, the electromagnetic wave reception method is employed. The electromagnetic wave reception method allows electric power of several watts or less to be transmitted in a band between medium waves and microwaves. However, the power use efficiency thereof is small. The electric field/magnetic field resonance method has been receiving attention as a method for supplying electric power with relatively high efficiency at a middle range on the order of several meters. Related techniques are disclosed in Non-patent document (A. Karalis, J. D. Joannopoulos, M. Soljacic, “Efficient wireless non-radiative mid-range energy transfer” ANNALS of PHYSICS Vol. 323, Jan. 2008, pp. 34-48), for example.
FIG. 1 is a diagram showing a wireless power transmission system according to a comparison technique. The wireless power transmission system 1r includes a wireless power supply transmitter 2r and a wireless power receiver 4r. The wireless power supply transmitter 2r includes a transmission coil LTX, a resonance capacitor CTX, and an AC power supply 10r. The wireless power receiver 4r includes a reception coil LRX, a resonance capacitor CRX, and a load 70.
The resonance frequency is an important factor in magnetic field (electric field) resonance power transmission. The resonance frequency of the transmitter side LC resonance circuit is represented by fTX=1/(2π√(LTX·CTX)). The resonance frequency of the receiver side LC resonance circuit is represented by fRX=1/(2π√(LRX−CRX)). Thus, in order to provide high-efficiency electric power transmission, there is a need to appropriately adjust the transmitter-side and receiver-side resonance frequencies and the frequency of the AC power supply 10r. However, in actuality, such resonance frequencies fluctuate depending on various kinds of factors. It is difficult for the power receiver side to tune the fluctuating resonance frequency based on the magnetic field (or electric field) itself as it has been transmitted from the power supply transmitter. This is because, in some cases, the resonance frequency detected by the power receiver side further changes depending on the resonance frequency and the phase conditions of the power receiver side.